The present invention relates to a fiber optic magnetic field sensor and more particularly to a fiber optic magnetic field sensor employing the Faraday effect.
For measuring a magnetic field in a highvoltage apparatus in which the electromagnetic induction noise, electrical insulating quality and the like are matters of great concern, a magnetic field sensor employing an optical method is more advantageous than a magnetic field sensor employing an electrical method from the viewpoint of measuring accuracy and cost.
FIGS. 1a and 1b show arrangements of conventional magnetic field sensors employing an optical method, respectively. Each of the sensors detects a magnetic field intensity as a light intensity signal by employing a Faraday material member 1, optical fibers 2, 3 for optical signal transmission, rod lenses 4, 5 and polarizing prisms 6, 7. The sensor shown in FIG. 1a is formed by arranging the above-mentioned elements in series, while the sensor shown in FIG. 1b is formed by arranging the elements into a reflection type sensor through the use of a mirror 8.
The operation of the magnetic field sensor shown in FIG. 1a will be described hereinunder. When no magnetic field H is applied to the Faraday material member 1, the plane of poralization of a light beam passing through the Faraday material member 1 does not rotate so that the light beam from a light source passes through the optical fiber 2, lens 4, poralizing prisms 6, 7 having the polarizing axes thereof making an angle of 45.degree. with respect to each other, lens 5 and optical fiber 3 and is sensed by a photo receiver connected to the optical fiber 3. In such a case, the reduction in quantity of light is only the portion of light attenuated in each of the above-mentioned elements.
On the other hand, in the case where the magnetic field H is applied to the Faraday material member 1, when the light beam linearly polarized by the polarizing prism 6 passes through the Faraday material member 1, the plane of poralization of the passing light beam rotates by an angle .phi. proportional to the intensity of the magnetic field H. The larger the rotational angle .phi., the larger the change in quantity of light passing through the polarizing prism 7. The amount of the change is detected by the photo receiver. Accordingly, the intensity of the magnetic field H can be measured from the change in quantity of light.
Both magnetic field sensors shown in FIGS. 1a and 1b require the rod lenses 4, 5 for transforming the light beam from the optical fiber 2 into a parallel light-ray beam and the polarizing prisms 6, 7 for linearly poralizing the light beam, e.g., Glan-Thomson prisms made of calcite, disposed at both the incident and emergent side of the Faraday material member 1, respectively, so that it is difficult to make the sensor unit more compact and lightweight. More specifically, both sensors shown in FIGS. 1a and 1b include the optical fibers 2, 3 having the rod lenses 4, 5 connected thereto respectively and hence require two lenses, together with two independent prisms completely separate from each other. Therefore, the arrangement of each of the sensors is unfavorable for miniaturization and reduction in cost. Moreover, the conventional polarizing prisms 6, 7 employ calcite as the material therefor, each using a set of two pieces of calcite triangularly cut in accordance with a specific crystalline orientation and cemented together. Consequently, miniaturization and mass production are difficult, so that the cost is inevitably extremely high.